1. Field of the Invention
The invention relates to liquid crystal display (LCD) apparatus and, in particular, to achieving uniform liquid-crystal cell gaps in such apparatus.
2. Description of Related Art
LCD apparatus typically include an active substrate on which an array of circuit elements (e.g. semiconductor devices, pixel electrodes, and row and column conductors) are deposited to define a display area, a window substrate including an optically-transparent portion disposed opposite the display area, and a seal surrounding the display area and compressed between the two substrates for containing the liquid crystal material. Typically, the active substrate also includes circuit elements positioned outside of the seal which are electrically connected to circuit elements in the display area via conductors passing through the seal.
Such an apparatus is described, for example, in U.S. Pat. No. 5,691,793, which is hereby incorporated by reference. As is stated in that patent, in fabricating an LCD apparatus it is important that the gap between the opposing substrates be as uniform as possible. In particular, the gaps between opposing electrodes on the two substrates, which define the liquid crystal cells for the different pixels, must be uniform. Variations in these gaps will adversely affect the quality of the image displayed. In order to make the gaps between the substrates uniform, the '793 patent proposes that gap-adjusting layers be deposited on shorter circuit elements to build them up to the same height as taller circuit elements.
The degree to which that approach achieves gap uniformity depends on the amount of tolerance buildup. In other words, the tolerance at each of the built-up circuit elements is the sum of the tolerances for the element itself and for the gap-adjusting layer. For any built-up element which itself includes multiple layers, the tolerance buildup can cause a gap error that is higher than tolerable for meeting the ever-increasing demands for LCDs with improved image quality.
Another known approach to achieving gap uniformity is that of scattering spacer elements having predetermined dimensions on one substrate before attaching the other substrate. This technique presents a variety of problems including:
the difficulty of evenly distributing the spacer elements on the substrate; PA1 the impracticality of selectively distributing spacer elements on active substrates supporting circuit elements with varying heights; PA1 the possibility that the spacer elements will land on display pixels and obstruct impinging image-forming light radiation. PA1 a first substrate having a supporting side on which are supported an array of pixel electrodes defining a display area; PA1 a second substrate having a covering side opposing the supporting side of the first substrate, the covering side bearing at least one counter electrode disposed opposite the pixel electrodes; PA1 a plurality of first spacer elements affixed to the first substrate at distributed locations within the display area, the first spacer elements having respective planarized surfaces at a uniform distance from the pixel electrodes; PA1 a plurality of second spacer elements positioned at distributed locations outside of the display area, each of the second spacer elements being affixed to one of the first and second substrates and having heights sized to support the second substrate relative to the first substrate such that the at least one counter electrode is parallel to the planarized surfaces of the first spacer elements, thereby establishing uniform cell gaps between the pixel electrodes and the at least one counter electrode; PA1 an adhesive for securing the first and second substrates together to maintain the second spacer elements in contact with first and second substrates.